Ground-source heat pump systems: The effect of variable pipe separation in ground heat exchangers

Closed loop ground-source heat pump (GSHP) systems use ground heat exchangers (GHEs) to transfer heat to and from the ground and efficiently provide clean and renewable energy for heating and cooling purposes. Vertical GHEs contain pipes with circulating fluid (loops), which transfer thermal energy between the ground and the fluid. One very common assumption made in designing GSHP systems is that, when installed, these loops remain evenly separated along the length of the GHE, something that due to the nature of construction is rarely true. This can result in thermal interference not accounted for in the design, leading to a potential negative impact on the performance of the system. This paper investigates the effect of this interference, using detailed numerical simulations to compare different geometries, modelling fixed and variable pipe separations. A comprehensive parametric analysis is conducted to identify some of the most influential design parameters and the potential consequences on running and capital costs. Amongst the key findings of this study is the importance of the borehole filling material, as a highly thermally conductive material can minimise these negative effects from the thermal interference by up to 60%. Moreover, potential increases in drilling (capital) costs of up to 24% are shown, while the potential increases in running costs due to the reduced efficiency were found to be relatively minor.